The present embodiments relate to an IR sensor and, more particularly, to and IR sensor with controllable sensitivity.
Infra-red (IR) sensors detect the IR radiation emitted from an object, and are used for day and night vision, non-contact temperature measurement in many industrial, security, and medical applications. These applications include aircraft imaging systems, human temperature sensors, surveillance systems, fire alarms, and night vision equipment.
IR sensors generally operate by detecting the differences in the thermal radiance of various objects in a scene. The difference is converted into an electrical signal, which is then processed and analyzed or displayed. Imaging cameras or radiometers, such as forward-looking IR (FLIR) instruments, utilize an array of IR sensors to provide a two-dimensional thermal image.
There are two distinctive sensor technologies: thermal detection and photon detection. Thermal detectors use secondary effects, such as the relation between conductivity, capacitance, expansion and detector temperature, to detect IR radiation. Thermal detectors include bolometers, thermocouples, thermopiles, and pyroelectric detectors. Photon detectors translate the photons directly into photoelectrons. The charge accumulated, the current flow, or the change in conductivity is proportional to the radiance of objects in the scenery viewed. Photon detectors generally have higher performance than comparable thermal detectors, with larger detectivities and smaller response times, but generally need to be cooled to cryogenic temperatures.
Systems based on cryogenically-cooled detectors have several advantages. The detector elements are usually smaller in size than comparable microbolometer detector elements. Shorter focal length lenses can be used to attain the same required spatial resolution. FLIR cameras based on cooled detectors generally have very good sensitivity, and reasonable physical dimensions even for long focal length optics, since high f-number devices may be used.
Four distinct generations of FLIR cameras have been designed, based on IR detector technologies developed during the last 30 years. The various generations are classified according to the number of elements contained in each group. First generation FLIR cameras used a small number of detectors and a mechanical scanner to generate a two-dimensional image. Today's fourth generation FLIR cameras contain two-dimensional focal plane array (FPA) of IR sensors that do not require any scanning mechanism.
After the sensor array is exposed to external radiation for a duration known as the exposure time, a readout circuit scans the sensor array and reads out the signal of the individual sensors composing the sensor array in a sequential manner. Readout circuits are classified into two modes, integrate then read (ITR) (also denoted snapshot readout) and integrate while read (IWR). The ITR approach performs detector exposure and sensor readout sequentially. The sensor array is exposed, read, exposed again, and so on. The IWR approach performs sensor exposure and readout in parallel, by storing the sensor signals from the previous exposure cycle in an analog or digital buffer. FPA detectors are commonly provided with an on-chip readout integrated circuit (ROIC).
In general, the readout time varies directly with the size of the sensor array. For example the typical readout time for a detector of 640 by 480 elements is about 10 milliseconds (10−2 second) while the readout time for a detector of 320 by 240 detector elements is only 2.5 milliseconds.
Reference is now made to FIG. 1, which is a simplified block diagram of a prior art FLIR camera. FLIR camera 100 contains an optical portion 110 which focuses IR radiation from an external scene onto sensor array 120 within IR detector 115. Optical portion 110 generally consists of one or more lenses, which may be adjustable to control focus positioning or to perform optical zoom. Sensor array 120 is a photon detector, therefore IR detector 115 is cooled to the cryogenic temperatures required for photon detection. Sensor array 120 is followed by readout circuit 130, which reads out the sensor array signals. A third detector component is the detector operation mode controller 135, which is a register containing the detector settings, such as sensor readout window size and location, operating mode (ITR/IWR), detector exposure time and additional variables such as bias current. The readout signal is processed by various processing elements, which in FIG. 1 are grouped together as processor 140. The processing elements generally perform tasks such as non-uniformity correction (NUC) and bad pixel replacement (BPR), as well as video processing to obtain a video signal from the sensor array signal output.
The current from an infrared detector may be subdivided into two parts: photocurrent and dark current. The photocurrent is the useful response of the detector, which results from absorption of infrared photons in the detector. These photons create charge carriers which can be collected as a photocurrent. Dark current is an undesired part of the detector current, which is present even if the detector is not illuminated. The origin of dark current is usually thermal excitation of charge carriers, a process that competes with photo excitation.
FLIR cameras with photon detector sensor arrays provide IR images with limited sensitivity. Photon detector SNR is limited by several factors. The detector can only absorb a limited number of photons. If the exposure time is too long many of the sensor array detectors saturate and a blank image is obtained. The sensor array must be discharged during readout often enough to prevent saturation. However, if the sensor array is discharged too frequently (due to a very short detector exposure time), the SNR for a single readout is limited by the noise which is present even in the absence of IR radiation. If the exposure time is too short, insufficient photons are collected and a dark image is obtained. Current FLIR cameras generally perform a single exposure/readout cycle per video frame. The image sensitivity is therefore limited by the ratio of the detector signal and the total noise associated with the process. The noise contains fluctuations associated with the signal itself (quantum noise), readout noise, fluctuations of the dark current, and so forth.
In an FPA-based FLIR camera, assuming an ideal optics, the spatial resolution of the image is determined by the number of pixels on the detector array. Common formats for commercial infrared detectors are 320×240 pixels (320 columns, 240 rows), and 640×480, with typical pitches between pixels in the range 20-50 um. FLIR cameras that have both a large field of view (FOV) and a high spatial resolution are usually based on large detector formats like 640×480, 1000×1000, or even larger, with a pitch size as small as 15 um and below. The read out time for such large detector formats is relatively long. In order to obtain a required frame rate for the video signal, the detector exposure time must often be limited, even for low f-number systems. The limited detection time results in insufficient light collection, which decreases the signal to noise ratio. Thus, high-resolution FLIR cameras with long readout times often have poor sensitivity.
There is thus a widely recognized need for, and it would be highly advantageous to have, an IR sensor and IR camera devoid of the above limitations.